Why Do ‘Super Moons’ Always Come in a Row? (Synopsis) [Starts With a Bang]

“The supermoon is a 16-inch pizza compared with a 15-inch pizza. It’s a slightly bigger moon; I ain’t using the adjective ‘supermoon.’” -Neil deGrasse Tyson

Earlier this month, the full Moon was the first “Supermoon” we’ve seen all year, where the bright full Moon coincided nearly perfectly with perigee, or the Moon’s closest approach to Earth. Yet it won’t be the last: November’s and December’s full Moons will also be Supermoons, appearing up to 14% bigger and 30% brighter than the full Moons from, say, March and April of this year.

A perigee full Moon compared with an apogee full Moon, where the former is 14% larger and the latter is 12% smaller than the other. Image credit: Wikimedia Commons user Tomruen under a c.c.a.-s.a.-3.0 license.

This isn’t a rarity, either: Supermoons always come in a row, and we usually get three in a row at that. Moreover, they don’t happen at the same time every year, but appear on a cycle that’s 411 days long. What’s the cause of this, and why does the Moon behave as it does?

The Moon takes a little over 27 days to orbit 360º around Earth, a little over 29 days to go from new Moon to new Moon again, but a total of 14 lunar cycles, or 411 days, to go from a full Perigee Moon to a full Perigee Moon again due to the motion of its elliptical orbit around the Sun. Image credit: Wikimedia Commons user Orion 8, under a c.c.a.-s.a.-3.0 license.